Theoretically Study The Two Dimensional High Carrier Mobility Materials For New Energy Applications

"Energy" and "environment" are the two serious problems faced by the humankind.The storage of traditional fossil fuels in the earth are limited and thus cannot meet the needs of human sustainable development.Exploiting,processing and use of fossil fuels will also bring harm to the environment.The energy problem may be solved by the following three approaches:? development of new clean energy,such as wind,water and solar energy.? perfecting energy devices,such as improving conversion/transformation/utilization efficiency and reducing costs of energy devices.? optimizing energy consumption devices,manufacturing low-power devices for example.The innovation of these new energy resources and new devices is inseparable from the development of materials.In recent years,with the exfoliation of monolayer graphene,a series of new two-dimensional materials have been experimentally and/or theoretically reported.The two-dimensional materials with high carrier mobility have found their wide applications in the energy absorption,transformation and storage fields.In this thesis,we focus on the potential roles of two-dimensional materials with high carrier mobility in the new energy field at the atomic scale and providing theoretical basis for relevant experimental researches.The two-dimensional materials considered in this thesis include PdSe2,two-dimensional transition-metal dichalcogenides in "Cairo"phase,SiC6 materials and Ti2C MXene.The properties of light absorption,the mechanism for electrolysis of water and processes in Li-S battery are mainly discussed.The main research contents and results are summarized as follows:1)An stable phase of palladium diselenide(PdSe2)monolayer that can be synthesized by selenizing Pd(111)surface is proposed.It has a moderate band gap of about 1.10 eV and a small in-plane stiffness with excellent ductility,which can sustains large tensile strain up to 35%.At the equilibrium structure,PdSe2 has a high electron motilities of about 9800-42000 cm2v-is-1.Additionally,tensile strain can modulate the band gap and consequently tune the light adsorption ability of PdSe2 monolayer.When the strain exceeds 14%,the two-dimensional PdSe2 will change from semiconductor to metal.The PdSe2 can absorb a wide range of visible light at equilibrium.The large tensile strain masks its absorption of visible light and enhances its absorption in infrared light.For example,under the tensile strains larger than 12%,the adsorption coefficients are almost zero in the energy region 1.0-2.0 eV,while the absorption for infrared light in the energy region 0.3-1.0 eV is very obvious.Under the compression strain,the light absorption of PdSe2 will shift to blue.This ultra soft two-dimensional PdSe2 material with good ductility and high carrier mobility is expected to be used as optoelectronic devices and solar energy.2)Two-dimensional transition-metal dichalcogenides with "Cairo" structures(cairo-TMDs)PdSe2 have advantages of good stability,high specific surface area and high carrier mobility.Based on the first-principles calculations,we predicted that the cairo-TMDs(TM=Ni,Pd,Pt;D=Se,S,Te)can serve as promising catalysts for oxygen evolution reaction(OER)in electrolysis of water.The theoretical lower limitation of OER overpotential is 0.16-0.245 V,which is comparable with the catalysts of noble Pt compounds.There is a linear relationship between the overpotential and the change of absorption energy.TMTe2 and TMSe2,compared with other TMD2,have more suitable adsorption energy for O/OH/OOH group,and thus a lower overpotential.NiD2 compounds containing non-precious metal Ni have lower overpotential than their counterparts PtD2.These predictions open an avenye for design of inexpensive non-precious metal OER materials.3)A novel SiC6 siligraphene composing of sp2 carbon and sp3 silicon atoms is proposed based on the first principle calculations.The unique hat-like structure leads to unusual mechanical properties,negative Poisson ratios.SiC6 siligraphene possesses a natural band gap of 0.73 eV and high carrier mobility.The hole and electron mobility of the siligraphene along[110]are in the order of 102 and 104 cm2V-1s-1,respectively.Whereas the hole transport along the[110]direction is blocked and electron mobility in this direction is in the order of 103 cm2V-1s-1.The electron mobility can be tuned to 3.155×104 cm2V-1s-1 by strain,but the hole always keeps a low migration rate(<102cm2v-1s-1).Stress can not only regulate the mobility of SiC6,but also change the bandgap of SiC6.The compression strain can change the SiC6 from the indirect bandgap into a direct band gap semiconductor.At the same time,a synthetic method is proposed theoretically.In addition,those properties make SiC6 siligraphene a versatile and promising 2D material for applications in nanomechanics and new energy field.4)The feasibility and physical mechanism of Ti2C MXene as sulfur host material for Li-S battery are studied.Using first-principles calculations,we demonstrated that the strong interaction between sulfur and pristine Ti2C MXene surfaces results in the decomposition of S8 molecules and surface sulfuration.The sulfuretted Ti2C MXene,however,has opportune interaction with polysulfide species,which can not only anchor the polysulfide species to suppress shuttle phenomenon but also prevent the decomposition of the polysulfides on the host material.The excellent metallic conductivity of sulfuretted Ti2C MXene is well preserved in the sulfuretted Ti2C MXene.Compared to graphene,Li in Li2S is more likely to diffuse on the S-Ti2C surface,which makes S-Ti2C lithium-sulfur batteries have a faster charge rate.The tunable interaction between polysulfides and host materials due to the self-functionalization effects,shedding light on the design of sulfur host materials for high performance Li-S batteries.